1. Field of the Invention
The present invention relates to a satellite tracking apparatus and control method for performing attitude control of a vehicle-mounted antenna for receiving a satellite broadcasting (or satellite communication receiving signals) and operating the antenna and more particularly to a satellite tracking apparatus and control method for quickly and accurately tracking a satellite in accordance with a moving direction of a vehicle with an antenna mounted to the vehicle, using an electronic tracking method and a mechanical tracking method.
2. Description of Related Art
To receive signals from a satellite, an antenna mounted to a mobile should be directed toward the satellite. For such purpose, an appropriate satellite tracking means is required. Typically, there are an open-loop tracking method using a sensor, a closed-loop tracking method using signals received from a satellite, and a hybrid tracking method employing both methods.
A step track method and a monopulse method are representative methods which search and hold a satellite using signals from the satellite. The open-loop tracking method is characterized by using a geomagnetic compass and a sensor such as a rate sensor.
Since airplanes and ships are usually equipped with navigation systems such as the navy navigation satellite system (NNSS) and the inertial navigation system (INS), the open-loop tracking method is usually employed. However, since signals may be blocked by tunnels or buildings, land vehicles employ the hybrid tracking method using the step track or monopulse method and an angle sensor together.
A conventional satellite tracking method comprises an initial satellite search mode, a tracking mode, and a blocking processing mode (or iterative tracking mode). In the initial satellite search mode, an antenna or beam is turned all around to detect a direction with a maximum signal level. In the tracking mode, a satellite is continuously tracked using a signal level, a monopulse phase signal, or data on vehicle's turning angle when the signal level exceeds a predetermined limit. In the blocking processing mode (or iterative tracking mode), the direction pointed to the satellite is maintained by using the data of a vehicle's turning angle sensor when signals of the satellite cannot be received because the vehicle is passing through a tunnel or buildings block the signals.
A conventional vehicle-mounted Ku-band satellite broadcasting receive antenna uses a pointing error, an azimuth obtained from a gyroscope, and AGC voltage when tracking a satellite. In an initial stage of searching the satellite, an azimuth is increased by 1.degree. while monitoring a receiving level represented by the AGC voltage and, when the signal level exceeds a limit value, Lo, a tracking operation is carried out. In the tracking operation, a pointing error is calculated using a monopulse phase difference and gyro data. If the receiving level is smaller than the limit value, Lo, a gyro control process is performed. Gyro data obtained from gyro control process is read and compared with a value of the receiving level just before the receiving level decreases, for calculating the pointing error of the antenna, thus maintaining a previous attitude of the antenna. Until a value of a timer exceeds a predetermined time, To, the procedure goes to the tracking process. If the receiving level is not restored to To, the procedure goes to a search process.
U.S. Pat. No. 449,671 discloses a vehicle-mounted Ku-band satellite broadcasting receive antenna similar to the above conventional art. It has been developed to accurately detect a pointing error by eliminating errors contained in an error signal obtained from a monopulse of the prior art. Obtaining a ratio of phase error signals represented by a sine and a cosine eliminates the error. Mean square values of a monopulse sine and phase error signal, an absolute error signal by a ratio of the mean square values, and gyro sensor data are used for the satellite tracking. In an initial satellite search process, if the mean square value is equal to or smaller than a predetermined limit value, the antenna is turned round for a given time. If the mean square value exceeds the predetermined value, the scanning is stopped and a peak detection is started. During the peak detection performed after the scanning of the antenna, a mean square is read and compared with the previous value. If the current value is larger than the previous one, the antenna is turned in a current direction. If not, the antenna is turned in an opposite direction, thereby directing the antenna to an orientation. The gyro data is then reset and angle data is read from the absolute error signal. After control the antenna, if a mean square value exceeds the predetermined limit value, consistency of the antenna to the orientation is determined high. After resetting the gyro data, a pointing error is obtained based upon the gyro data. In the blocking process, if the mean square value is smaller than the specified limit value indicating signal blocking, the pointing error in the gyro data is read to control the antenna. If the mean square exceeds the predetermined limit value, the antenna is controlled based upon an error signal.
U.S. Pat. No. 5,166,693 is provided for L-band mobile satellite communication. In this patent, satellite tracking control comprises search of satellite direction, on-turning beam control, on-nonturning beam control, and on-blocking beam control. A receiving level is read and compared with a switching level. If the receiving level is lower than the switching level, it is compared with a blocking level. If the receiving level is lower than the blocking level, the procedure goes to a blocking mode to perform the tracking based upon an angle obtained by an angle sensor. If the receiving level is equal to or higher than the blocking level, an angle obtained by the angle sensor is read and compared with the previous value to determine a state of turn. During the satellite search, a receiving level is read after changing the direction of a beam. If the receiving level exceeds a maximum receiving level, it is memorized as a new maximum receiving level and a current direction of the beam is memorized. Thereafter, scanning is performed in all direction. During the onblocking beam control, data of the angle sensor is read to determine a turning angle. If the turning angle exceeds a reference angle, the beam is changed to an adjacent beam and then a receiving level is read. If the receiving level is equal to or higher than that the switching level, it is maintained. If the receiving level is lower than the switching level, a timer is checked. Until a predetermined time has passed, the previous steps are repeated. Thereafter, the procedure goes to a satellite search mode. During the on-nonturning beam control, if the receiving level is higher the blocking level and lower than the switching level, the beam is changed to a leftward adjacent beam. A receiving level detected after changing the beam is compared with the previous level. If the current level exceeds the previous one, left turn is determined. If not, the beam is changed to a rightward adjacent beam. A current receiving level is then compared with the previous receiving level. If the current level exceeds the previous one, the current receiving level is read and compared with the switching level. If not, the beam is returned to an original direction. During the on-turning beam control, the direction of turn is determined and the beam is scanned. A current receiving level is compared with the previous level. If the current level exceeds the previous level, the current receiving level is read and compared with the switching level. If not, the beam is returned to the original direction.
The following problems occur when such satellite tracking method using the conventional vehicle-mounted antenna is actually applied to a vehicle-mounted satellite broadcasting receive antenna system.
(1) When an azimuth is only mechanically controlled according to the monopulse track method corresponding to the closed-loop tracking method, rapid and accurate control cannot be achieved with the conventional techniques.
(2) It is difficult to implement a satellite antenna tracking system of high gain required for satellite reception since beam efficiency decreases in a full-electronic tracking method. Besides, the structure is complicated.
(3) When a vehicle changes its moving direction with a large pointing error, it is difficult to realize an accurate capture for a short search time when searching the direction of a satellite.
(4) In case of an array antenna employing a hybrid antenna system, a beam steering controller function is installed in a rotating body and a fixed body includes a central processing unit carrying out a main algorithm. Therefore, serial data communication and control is achieved through a rotary joint. This makes rapid control impossible.
(5) When mechanically controlling an azimuth and using a step motor, power efficiency with respect to a torque is low and high cost is required although control is conveniently carried out. When using a direct current servo motor for the control of the azimuth instead, a response characteristic becomes unstable during general rapid response control while the response characteristic becomes slow during stable control. Consequently, it is difficult to achieve stable and rapid response control.